Dosimetric Comparison of VMAT Alone and VMAT with HDR Brachytherapy Boost Using Clinical and Biological Dose Models in Localized Prostate Cancer

Abstract

Background: Combining external beam radiotherapy (EBRT) with high-dose-rate (HDR) brachytherapy (BT) enables biologically effective dose escalation in prostate cancer. However, comparative evaluation of such regimens using radiobiological modeling remains limited.

Methods: Dose regimens based on clinical practice were analyzed using α/β values of 1.5 and 3 Gy for the prostate. Ten patients with available planning CT, pelvic MRI, and ultrasound-guided BT plans were retrospectively evaluated. Physical and biological dose distributions were recalculated for various EBRT and HDR-BT combinations. Biological effective dose (BED) values were determined for the prostate and organs at risk (OARs: anterior rectal wall, bladder base, urethra). Regimens yielding the highest ΔBED between prostate and OARs were considered most favorable.

Results: All regimens met clinical dose constraints. The most favorable ΔBED profiles for bladder and rectum were observed with HDR-BT regimens (2 × 15 Gy) combined with either 23 × 2 Gy or 15 × 2.5 Gy EBRT, independent of the assumed α/β value. EBRT-only regimens achieved superior urethral sparing, while higher HDR doses led to increased urethral exposure.

Conclusions: This study underscores the value of radiobiological modeling in differentiating and optimizing prostate cancer radiotherapy strategies. While the trade-offs between dose escalation and OAR sparing are clinically known, our biologically driven analysis provides a more quantitative foundation for selecting and tailoring combined EBRT/HDR-BT regimens in practice.

Keywords: biologically effective dose (BED); dose escalation; external beam radiotherapy (EBRT); high-dose-rate brachytherapy (HDR-BT); hypofractionation; organs at risk (OARs); prostate cancer; radiotherapy.

Figure 1

Example of dose distribution in high-dose-rate (HDR) brachytherapy and external beam radiotherapy (EBRT) for prostate cancer treatment of patient #6. Top row (a1–c1): Ultrasound-based images showing dose distribution in HDR brachytherapy with color-coded isodose lines (see color legend (d1)). Coronal (a1), sagittal (b1), and transverse (c1) views illustrate highly heterogeneous intraprostatic dose distributions with focal high-dose regions (>200% of the prescribed dose). Note the sparing of the urethra (yellow contour in (a1,c1)) and the rectum (green contour in (b1,c1)). Bottom row (a2–c2): CT-based dose distribution in EBRT using the VMAT technique in corresponding planes, showing a more homogeneous dose coverage around the prescribed target dose (color legend (d2)). Note that organs at risk receive a higher percentage of the prescribed dose (urethra: green; rectum: blue; bladder: yellow).

Figure 2

Biologically effective doses (BEDs) for the three most exposed subvolumes of the bladder (a), rectum (b), and urethra (c) are shown across different fractionation schemes, all normalized to an equivalent BED in the prostate target assuming an α/β ratio of 1.5 Gy. Organ-specific α/β values were assumed as follows: 2.0 Gy for the bladder, 3.0 Gy for the rectum, and 1.0 Gy for the urethra. Description of the box plots: The thick horizontal line inside the rectangular box indicates the median of the data distribution. The lower edge of the gray-shaded box represents the first quartile (Q1), and the upper edge represents the third quartile (Q3). The vertical length of the box (i.e., Q3–Q1) is known as the interquartile range (IQR). The whiskers on either side extend to values up to 1.5 times the IQR from Q1 or Q3. Data points located more than 1.5 but not more than 3.0 times the IQR from Q1 or Q3 are considered outliers (depicted as circles), while those farther than 3.0 times the IQR are classified as extreme values (typically shown as asterisks—none appear here).

Figure 3

Comparison of various fractionation schemes regarding the difference in biologically effective dose (ΔBED) between the target prostate volume (assuming an α/β ratio of 1.5) and the most exposed portion of the bladder (α/β ratio of 2.0): (a) referring to the highest exposed 2 ccm, (b) to the highest exposed 0.1 ccm. A detailed explanation of the boxplots is provided in the legend of Figure 2.

Figure 4

Comparison of various fractionation schemes regarding the difference in biologically effective dose (ΔBED) between the target prostate volume (assuming an α/β ratio of 1.5) and the most exposed portion of the urethra (α/β ratio of 1.0): (a) referring to the highest exposed 1 ccm, (b) to the minimally exposed volume (Dmin) 0.1 ccm. A detailed explanation of the boxplots is provided in the legend of Figure 2.